Isolation is often provided in a power converter which runs off an AC line. The associated control logic or circuitry may receive its power from the primary side (i.e., AC line) or the secondary side or output side (See FIGS. 1A and 1B). If on the secondary side, a bias transformer provides initial power. Once the power converter starts, it then provides additional power for the control circuitry.
With increasing demand for lower cost and higher power density, the control logic or circuitry of a power converter is more often being placed on the primary AC line, where it can be powered initially without an isolated or bias transformer. In such a situation, an extra winding off (see FIG. 1B) the power converter transformer provides additional power; U.S. Pat. No. 4,694,386 to de Sartre is representative of this arrangement. The output voltage (or current) is then sensed by additional circuitry located on the secondary side and a signal is sent back through a small pulse transformer to the primary side control circuitry (See U.S. Pat. No. 4,717,994 to Diaz et al., and assigned to the assignee of the present invention).
Those skilled in the art will appreciate the fact that, when transformers are used, the expense of the associated circuitry increases. Even when a transformer power supply uses part of an existing transformer (i.e., additional secondary windings) the core mass increases and the cost of manufacture increases, although not as much as in the former case.
In switching or switch-mode power supplies, some measures are required to avoid impressing excessive induced voltages across the switching device (usually a semi-conductor or solid-state switch) when it transfers from the closed or conducting state to an open or off state. If the switched current is suddenly terminated, the voltage induced in the primary side of the transformer by the sudden collapse of current is capable of reaching enormous values (e.g., 200 volt spike). Unless something is done, the induced voltage can well exceed the break down voltage of the switching semi-conductor.
One solution has been to shunt the primary winding of the transformer or the switching device (or both) with some means of absorbing the leakage inductance energy in the transformer (See U.S. Pat. No. 4,172,277 to Pinson). Typically, such a shunt includes a capacitor and a diode for establishing a current loop whereby the transformer primary current is used to charge a capacitor when the switch is open. When the switch again closes, the energy stored in the capacitor is dissipated through a discharge path.
Many power converters which use solid-state switches (i.e., bi-polars, FET's, SCR's etc.) utilize "snubbers" to minimize power stress due to inductive load. Typically, the stored energy of the snubber is converted to heat energy or dissipated through a resistor. In some converters, the energy of the snubber is diverted to the output of the converter so as to perform non-dissipative snubbing (See U.S. Pat. No. 4,675,796 to Gautherin et al.; U.S. Pat. No, 4,561,046 to Kuster; U.S. Pat. No. 4,734, 636 to Stevens; U.S. Pat. No. 4,063,306 to Perkins et al.; and U.S. Pat. No. 4,607,322 to Henderson).
Therefore, it is one of the objects of the present invention to provide a smaller, more efficient, and lower cost power converter. In particular, it is one of the objects of this invention to use existing components, insofar as possible, so as to provide a dual function and thereby obtain two effects from the same circuitry or components.